(1) Field of the Invention
The present invention relates to a polymer electrolyte fuel cell. More particularly, the present invention relates to a polymer electrolyte fuel cell which is operated while a humidifying agent is supplied to a polymer electrolyte membrane together with either or both of a fuel gas and an oxidant.
(2) Description of the Prior Art
Many of commercially practical polymer electrolyte fuel cells are a stack of a plurality of cell units each of which includes a cell sandwiched by a pair of ribbed plates each having a plurality of parallel ribs with a certain pitch on a surface, where the cell is composed of an anode, a cathode, and a polymer electrolyte membrane, the polymer electrolyte membrane being disposed between the anode and the cathode.
Spaces formed between each adjacent ribs and the cathode or anode are used as channels. During operation of the polymer electrolyte fuel cell, anode-side channels are supplied with a fuel gas (e.g., hydrogen) and cathode-side channels are supplied with air (O2) as oxidant. During such an operation, hydrogen decomposes into hydrogen ions (H+) and electrons (exe2x88x92) at the anode. The hydrogen ions (H+) pass through the polymer electrolyte membrane, an electrolyte, to reach the cathode. The electrons (exe2x88x92) flow out to an external circuit. At the cathode, water (H2O) is generated by reaction between the oxygen (O2), the hydrogen ions (H+) coming from the anode, and the electrons (exe2x88x92) coming from the anode via the external circuit. This provides electric energy.
As described above,the hydrogen ions (H+) pass through the polymer electrolyte membrane to reach the cathode. This is possible because the polymer electrolyte membrane has a property to selectively pass (diffuse) hydrogen ions in hydration (H+(XH2O)). Therefore, conventional methods have been used to humidify the polymer electrolyte membrane. Among such methods are: a method in which a fuel gas having been humidified is supplied so as to humidify the polymer electrolyte membrane; and a method in which a fuel gas and water separately supplied to the anode-side channels flow together through the channels so that the fuel gas is effectively supplied to the anode, the polymer electrolyte membrane is effectively humidified, and the cell is cooled.
Meanwhile, as the polymer electrolyte membrane becomes dry, the ion permeability of the membrane decreases, increasing internal resistence and reducing the electric power generation efficiency.
Accordingly, to make full use of the electric power generation capability, the polymer electrolyte membrane needs to be humidified in its entirety. That is to say, of the polymer electrolyte membrane, not only a part facing the channels, but a part facing highest ends of the ribbed plate should sufficiently be humidified.
Conventionally, the ribbed plate is made of a carbon porous material. With this construction, water permeates into the ribbed plate sufficiently since the ribbed plate is porous. That is, the polymer electrolyte membrane is humidified in its entirety, and the part facing highest ends of the ribbed plate is also sufficiently humidified.
However, a typical method for manufacturing the ribbed plate has low manufacturability, resulting in a high manufacturing cost. Typically, the ribbled plate is manufactured with the following procedure. manufacture paper from carbon fiber chips; harden the paper with phenol resin to manufacture sheets of paper; stack the sheets of pater and bake the stack to obtain a plate made of a carbon porous material; and cut the plate to each piece of the ribbed plate.
As a method with which the manufacturability can be increased, a method in which the ribbed plate is formed from a mixture of carbon and resin using a mold is currently considered. However, molded products have dense construction. With such construction, the surface of molded ribbed plates becomes water-repellent, and a part of the polymer electrolyte membrane facing highest ends of the ribbed plate is not sufficiently humidified.
It is therefore the first object of the present invention to provide a polymer electrolyte fuel cell for practical use in which the polymer electrolyte membrane is maintained to be humidified entirely even if a dense material is used for the ribbed plate.
It is the second object of the present invention to provide a polymer electrolyte fuel cell in which the polymer electrolyte membrane is sufficiently humidified, channels formed between the ribbed plate and electrodes are not easily blocked by water, and diffusion of gas flowing through the channels into the electrolyte membrane is secured.
It is the third object of the present invention to provide a polymer electrolyte fuel cell which can be manufactured at a low cost.
It is the fourth object of the present invention to provide a polymer electrolyte fuel cell which includes a ribbed plate which: has high precision in shape; and is manufactured using a mold from a material having electric conductivity required for the cell performance.
The first object of the present invention is fulfilled by a polymer electrolyte fuel cell comprising: a cell including an electrolyte membrane and two electrodes which are disposed on two surfaces of the electrolyte membrane; a first ribbed plate having a plurality of ribs on a surface facing one of the two electrodes of the cell; and a second ribbed plate having a plurality of ribs on a surface facing the other of the two electrodes, where a water-retaining layer having a certain range of ability to retain water is formed to coat one or more tops of the plurality of ribs of at least one of the first ribbed plate and the second ribbed plate.
The second object of the present invention is fulfilled by the above polymer electrolyte fuel cell in which the water-retaining layer has ability to retain water per unit active area of the two electrodes in a range of 0.002 g/cm2 to 0.035 g/cm2, or a range of 0.01 g/cm2 to 0.03 g/cm2.
The third object of the present invention is fulfilled by the above polymer electrolyte fuel cell in which the ribbed plate with the water-retaining layer is formed using a mold from a resin-carbon mixture without baking or is formed from a metal.
The fourth object of the present invention is fulfilled by the above polymer electrolyte fuel cell in which the ribbed plate coated with the water-retaining layer is formed from a resin-carbon mixture without adding heat using a mold, and ratio of resin to carbon in this resin-carbon mixture is 10% to 20% by weight on a percentage basis.